1. Field of The Invention
This invention generally relates to a novel process for making polysilazanes substantially free of halide impurities. More particularly, this invention relates to a liquid phase process for making polysilazanes via the transamination and condensation of certain aminosilanes, particularly tris-dimethylaminosilane and/or vinyl-tris (dimethylamino)silane, with ammonia and/or monomethylamine in the presence of a Bronsted acid catalyst. The resulting polysilazane polymers range from fluids and resins to insoluble powders. These silazane polymers can be used as precursors for silicon based ceramic materials, binders or fibers.
2. Prior Art
Silicon nitride, Si.sub.3 N.sub.4, is currently a material of great potential and interest in that its ceramic properties include high temperature stability, chemical inertness, oxidation resistance and extreme hardness. In the past, silicon nitride has been prepared by a variety of methods, including the reaction of silicon metal with gaseous N2 and/or NH.sub.3, giving reaction-bonded silicon nitride, ("RSBN"). RSBN is useful for forming bulk parts via methods of powder metallurgy; for example hot pressing, sintering, casting or extruding.
Another method of preparing silicon nitride is via chemical vapor deposition ("CVD"). The reaction of H.sub.4-x SiCl.sub.x, where x=0, 1, 2, 3 or 4, with NH.sub.3 in the gas phase at high temperatures results in the preparation of high purity silicon nitride. The purity of the product is related to the purity of the reactant gases. The CVD method is a fundamental process for fabricating electronic materials and devices which require high purity silicon nitride.
Recently, routes have been developed for making silicon containing ceramics from the pyrolysis of polyorganosilazanes, such as described in U.S. Pat. No. 3,853,567 to Verbeek et al. and U.S. Pat. No. 3,892,583 to Winter et al.
Typically, these silazane polymers are synthesized from the reaction of halogen containing silanes with ammonia [J. Am. Ceramic. Soc. 67 132 (1984)], or by the reaction of halogen containing alkylsilanes with ammonia [J. Poly. Sci. A2 3179-3189 (1964)], primary amines [Acta. Chem. Scand. 13 29-34 (1959)], diamines [J. Poly. Sci. A2 44-55 (1964)] or silazanes. Because the starting reactant is a halogen containing silane or halogen containing alkylsilane, this amination step results in the synthesis of an amine hydrohalogen byproduct in addition to the desired organosilazane. Two representative reactions are shown below: EQU HSiCl.sub.3 +6NH.sub.3 .fwdarw.HSi(NH.sub.2).sub.3 +3 NH.sub.4 Cl EQU SiCl.sub.4 +8NH.sub.3 .fwdarw.Si(NH.sub.2).sub.4 +4 NH.sub.4 Cl
Recent U.S. Pat. Nos. 4,535,007, 4,540,803 and 4,543,344 to Cannady teach methods for preparing silicon nitride-containing ceramics by high temperature firing of a R.sub.3 SiNH containing silazane polymer. Cannady makes clear that in these patents the R.sub.3 SiNH-containing hydrosilazane polymers are derived from halogen containing silane or halogen containing alkylsilane reactants.
The prior art also teaches that lower boiling amines, i.e. NH.sub.3, can be used to transaminate the dimethylamino groups on (CH.sub.3).sub.3 SiN(CH.sub.3).sub.2 to give trimethylsilylamine, (CH.sub.3).sub.3 SiNH.sub.2, which ultimately decomposes at room temperature to give hexamethyldisilazane, ((CH.sub.3).sub.3 Si).sub.2 NH, Wiberg and Uhlenbrock, Chem. Ber. 104 pp. 2643-2645 (1971). This work however does not teach the production of silazane polymers and does not use acid catalysis.
Consequently, except when silane, SiH.sub.4, is used as a starting reactant, there is no teaching in the prior art for making polysilazane polymers free of ammonium hydrogen halide containing byproducts. The use of silane, however, is inherently dangerous due to the explosive reactivity of SiH.sub.4 and O.sub.2. In addition, those processes in the prior art involving the use of halogen containing silanes to prepare polysilazanes require an imperfect and time-consuming filtration and/or calcination step to substantially eliminate the viscous ammonium hydrogen halide byproducts created by the reaction. These processes thus require a lengthy and difficult filtration process step and/or a time consuming and energy intensive calcination process step. Moreover, these process steps are not always completely satisfactory because of the difficulties inherent in attempting to separate amine hydrohalogen byproducts from the liquid or solid silazane polymer products. Thus, there is a need in the art for a safer, more economic, more expedient and less energy intensive process for making silazane polymers substantially free of halide impurities.